The present invention relates to an apparatus which processes fuel vapor evaporated in a fuel tank in an internal combustion engine.
In order to prevent the fuel vapor evaporated in the fuel tank from diffusing in an atmospheric air, various evaporated fuel processing apparatus, in which the fuel vapor is fed into a canister filled with an absorbent to absorb and recover the vapor, have been conventionally proposed.
For example, one which is disclosed in the official gazette of Japanese Patent SHO 53-4171 comprises an evaporated fuel condensation tank disposed between a fuel tank and a canister to condense evaporated fuel before entering into the canister. And, in the case that a part of the evaporated fuel is not condensed in the evaporated fuel condensation tank, such a evaporated fuel not condensed in the evaporated fuel condensation tank is trapped in the canister. Furthermore, there is provided a nonreturn valve in a connecting pipe communicating to the canister so that, when the absorption in the canister is saturated, this nonreturn valve can prevent the fuel vapor from flowing backward to the fuel tank from the canister.
FIG. 1 shows such a conventional vent system that allows the fuel vapor to flow toward the canister from the fuel tank but prohibits fresh air to enter into the fuel tank from canister side or through a fuel filler cap clearance, etc.
A vent pipe 02 extending from a gaseous phase of a fuel tank 01 is connected to a charcoal canister 03, and there is provided a nonreturn valve 04 in an intermediate portion of the vent pipe 02 which allows one-way gas flow so that the fuel vapor can flow out of the fuel tank 01 along the pipe.
The nonreturn valve 04 is opened by a predetermined differential pressure ( for example 25 mmHg ). On the other hand, no negative pressure valve which allows gas flow toward a negative pressure side is provided on a fuel filler cap 06 provided at an opening of an oil feeding pipe 05 of the fuel tank 01. The fuel tank 01 has a strength sufficient to bear a negative pressure of -300 to -350 mmHg in gauge pressure.
In accordance with this vent system, even if an ambient temperature around the fuel tank is repeatedly increased and decreased within a predetermined temperature zone, the fuel vapor flows to the charcoal canister 03 only once if the vapor pressure of the evaporated fuel exceeds a set value of the nonreturn valve 04 when the temperature rises for the first time. And, after this first temperature rise, the fuel tank 01 no longer respires, therefore the evaporated fuel is surely prevented from leaking out of the fuel tank, or an atmospheric air is barred from entering into the fuel tank.
However, after an engine is stopped, when an inner pressure of the fuel tank 01 falls to be negative on account of a temperature fall, it was feared that a fuel pump becomes likely to cause a vapor look phenomenon. Or the negative pressure in the fuel tank 01 increases as the fuel in the fuel tank is consumed, therefore it was also feared that it causes an undesirable reduction of fuel feeding rate since the fuel pump cannot perform its normal function sufficiently.
Moreover, there was such a problem that it becomes hard to open the fuel filler cap 06 because the fuel cap 06 sticks fast when the negative pressure in the fuel tank 01 becomes large.
The present invention is attained in view of such problems, and the purpose of the present invention is to provide an evaporated fuel processing apparatus which is capable of preventing the fuel feeding characteristic from unexpectedly changing due to the negative pressure caused in the fuel tank.